Add a status navigation bar button to toggle between available and unavailable when on shift

This change adds a navigation bar button to the client and schedule table view that allows the user to toggle between being available and unavailable.

Added PPRStatusBarButtonItem
- A UIBarButtonItem subclass to display the shift status and toggle availability.

Added PPRStatusShowingTableViewController
- Looks for a PPRStatusBarButtonItem in the navigation bar and adds it as an observer for shift change notifications
- Connects the button action to a routine that toggles the availability.

PPRClientTableViewController now is a subclass of PPRStatusShowingTableViewController
PPRScheduleTableViewController now a subclass of PPRStatusShowingTableViewController
Modified Main.storyboard
- add status buttons to client table view and schedule table view
Merge branch 'shift_status_button'
Merge Pull Request 20